Hash iterators: Check for concurrent modification and fix 1 place where it happened

Source/core/animation/DocumentTimeline.h

 /*
  * Copyright (C) 2013 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 79 matching lines @@
 
     void detachFromDocument();
 
 protected:
     DocumentTimeline(Document*, PassOwnPtr<PlatformTiming>);
 
 private:
     double m_zeroTime;
     Document* m_document;
     // AnimationPlayers which will be updated on the next frame
-    // i.e. current, in effect, or had timing changed
-    HashSet<RefPtr<AnimationPlayer> > m_playersNeedingUpdate;
+    // i.e. current, in effect, or had timing changed. This needs
+    // to be a collection that can be updated while we iterate
+    // over it (this happens during pauseAnimationsForTesting).
+    ListHashSet<RefPtr<AnimationPlayer> > m_playersNeedingUpdate;
     HashSet<AnimationPlayer*> m_players;
     bool m_hasOutdatedAnimationPlayer;
 
     void wake();
 
     friend class SMILTimeContainer;
     static const double s_minimumDelay;
 
     OwnPtr<PlatformTiming> m_timing;
 
@@ skipping 17 matching lines @@
         Timer<DocumentTimelineTiming> m_timer;
 
     };
 
     friend class AnimationDocumentTimelineTest;
 };
 
 } // namespace
 
 #endif